Photographic layers sensitive to blue light for use in color photographic materials typically contain a yellow coupler which, on reaction with an oxidized arylamine developer, forms a yellow dye. Most commercially available photographic films contain pivaloyl- or benzoylacetanilide yellow couplers. These classes of couplers are, in general, satisfactory, but a person skilled in the art will be aware that even the best examples of these classes are a compromise between coupler activity on the one hand and dye stability on the other. Dodecyl 4-chloro-3-2-(1-benzyl-5-ethoxy-2,4-dioxoimidazol-3-yl)-2-(2,2-dimethylpr opanoyl)-acetamido!benzoate, for example, has good dye stability, but has a relatively poor contrast; dodecyl 4-chloro-3-2-(1-benzyl-5-ethoxy-2,4-dioxoimidazolin-3-yl)-2-(4-methoxyben zoyl)acetamido!benzoate, on the other hand, has a relatively good contrast but poor dye stability. There has therefore been a requirement to find further classes of yellow couplers. In each class discovered there is a chance that one or more examples may exhibit a combination of parameters which is better than the yellow couplers hitherto available in the prior art.
UK patent application Nos. 9513108.2 and 9513114.0 respectively disclose further classes of yellow dye-forming couplers, namely thenoylacetamides and pyrroloylacetamides, which can have properties which are at least comparable to the commercially available couplers in terms of coupler activity, contrast and/or light and dark/wet fade properties.
A further class of yellow dye-forming couplers is disclosed in U.S. Pat. No. 5,338,654 and U.S. Pat. No. 2,500,487 which disclose certain malonic acid half ester, half amides, which are useful as photographic couplers, providing yellow image dyes with good stability and having excellent dye characteristics such as high color density and sharpness.
A single method for preparing these couplers is disclosed therein which involves the addition of the required coupling-off group to the halogenated half ester, half amide in a yield of 70-80%. However there is no disclosure of the preparation of the starting material for this reaction. Attempts to prepare the halogenated couplers of this type by a number of standard methods used for the synthesis of conventional acylacetanilide couplers have failed to produce pure samples of the chloro or bromo couplers. Extensive quantities of starting material and dichloro or dibromo couplers were formed in all cases which made the ensuing metathesis of halogen for the coupling-off group very difficult to carry out.